Conventionally, an aluminum brazing sheet for brazing has been employed in a header, a side plate, and the like of an automobile radiator, and an Al—Mn based aluminum alloy such as JIS3003 has been employed as a core material. Also, an Al—Si based aluminum alloy such as JIS4045 or JIS4343 has been employed as a brazing material, and an Al—Zn based aluminum alloy has been employed as a cladding material which serves as a sacrificial anode. However, the post-braze strength of the brazing sheet employing an Al—Mn based alloy such as JIS3003 serving as a core material is about 110 N/mm2, which is not sufficient for a brazing sheet. In addition, the above brazing sheet does not have sufficient corrosion resistance. In order to improve the post-braze strength, the addition of Mg to a core material is effective. However, when such a material is employed for brazing together with a flux which generates a brittle compound through reacting with Mg, particularly, as in Nocolok brazing, the brazeability of the brazing sheet in which Mg is added to a core material is significantly deteriorated. Therefore, the addition of Mg to a core material is not preferable.
On the other hand, about 2 mass % of Mg has been added to a cladding material (see Japanese Patent Laid-Open Publications Nos. 2000-210787 and 2000-87163). In this case, Mg added to a cladding material diffuses from the cladding material to a core material during heating for brazing. At the same time, Si diffuses from a brazing material to the core material and is combined with Mg to form Mg2Si. Therefore, the strength can be improved through the addition of Mg to a cladding material.
However, when the above cladding material with high Mg content is laminated with a core material through clad-rolling processing, a blister may be formed on the product surface due to pressure adhesion failure. This results in lower yields of products, so that the productivity is lowered.